In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within the 3rd Generation Partnership Project (3GPP). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Serving Gateways, or SGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and SGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
Cellular Circuit-Switched (CS) telephony was introduced in the first generation of mobile networks. Since then CS telephony has become the largest service in the world with approximately 4 billion subscriptions sold. Even today, the main part of the mobile operator's revenue comes from the CS telephony service (including Short Message Services (SMS)), and the 2G GSM networks still dominate the world in terms of subscriptions. 3G subscriptions are increasing in volume, but that increase is less in part because of users with handheld mobile terminals migrating from 2G to 3G and more as a result of mobile broadband implemented via dongles or embedded chipsets in laptops.
The long-term evolution (LTE) project within 3GPP aims to further improve the 3G standard to, among other things, provide even better mobile broadband to the end-users (higher throughput, lower round-trip-times, etc.).
A common view in the telecommunication industry is that the future networks will be all-IP networks. Based on this assumption, the CS domain in was removed in the LTE work. As a result, the telephony service cannot be used by a 3GPP Release 8 compliant LTE terminal, unless one of the following four things is done:    (1) Implement circuit switched (CS) fallback (CSFB), so that an LTE terminal falls back to 2G GSM when telephony service is used.    (2) Implement 3GPP IP Multimedia Subsystem (IMS)/Multimedia Telephony (MMTel), which is a simulated CS telephony service provided over IP and IMS that inter-works with the Public Switched Telephone Network (PSTN)/Public Land Mobile Network (PLMN).    (3) Implement a tunneling solution with Unlicensed Mobile Access (UMA)/Generic Access Network (GAN) over LTE where the CS service is encapsulated into an IP tunnel.    (4) Implement a proprietary Voice over IP (VoIP) solution with PSTN/PLMN interworking.
All of these four possibilities have drawbacks. In deployed GSM networks that do not have Dual Transfer Mode (DTM) capabilities; CS and Packet Switched (PS) services cannot be used in parallel. Hence, all PS services running prior to a call to or from a terminal using Circuit Switched Fallback (CSFB) are put on hold or are terminated. If the GSM network has DTM, the PS performance will be greatly reduced (from 10's of Mbps to 10's to 100's of kbps). One drawback with the CS fallback approach is that when calling or being called and the terminal is falling back to GSM and the CS service from LTE. Circuit Switched Fallback (CSFB) also prolongs call set-up time.
The IMS/MMTel approach uses a completely new core/service layer that is IMS based. This provides new possibilities to enhance the service but also comes with the drawback of a financial hurdle for the operator to overcome. A new core network drives capital expenditures (CAPEX), and integration of that core network drives an initial operating expenditures (OPEX) increase. Further, the IMS/MMTel approach needs features implemented in the terminals and the legacy CS network in order to handle voice handover to/from the 2G/3G CS telephony service.
Using UMA/GAN over LTE is not a standardized solution so a drawback is that it is a proprietary solution which may make terminal availability a problem. It also adds additional functions to the core/service layer in both the network and terminal, e.g., a GAN controller in the network and GAN protocols in the UE terminal.
The proprietary VoIP approach, if operator controlled, comes with the same drawbacks as for the IMS/MMTel (new core/service layer) approach along with the difficulties associated with it being proprietary and handover to 2G/3G CS may not be supported.
There is yet a further solution for using a legacy CS telephony service with a wireless terminal such as a 3GPP release 8-compliant LTE terminal. In that further solution, also known as a type of Access Division Multiplexing (ADM), transmissions of GSM CS voice are interleaved in between LTE transmissions. See, e.g., PCT/SE2007/000358, which is incorporated herein by reference. In one example implementation of such an ADM solution a wireless terminal simultaneously communicates with two TDMA-based radio systems, e.g., the wireless terminal can maintain communications paths to both systems by means of alternating in time its communication between the two systems. The toggling between the two systems is on a time scale small enough to effectively yield a simultaneous communication between the two systems.
This further solution attempts to achieve a good PS connection in parallel with the telephony service when in LTE coverage but still reusing the legacy CS core and deployed GSM network for the telephony service to reduce costs but still maintain good coverage for the telephony service.
This further or ADM solution may be implemented in several ways. A first example implementation, illustrated in FIG. 1A, is a fully UE centric solution where no coordination is needed between the GSM CS core and a LTE PS core. A second example implementation, illustrated by FIG. 1B, is a network assisted solution which can either be based on circuit switched fallback (CSFB), or a solution that only reuses paging over LTE.
From a radio perspective, the solution can be realized in any of three different ways: As a first example radio realized embodiment illustrated in FIG. 2A, the LTE transmissions could be multiplexed with the GSM transmissions on a GSM TDMA frame level. This first example solution requires that the GSM CS telephony service only use the half rate codec. When GSM is running at half rate, then every second GSM TDMA frame is not used by the user.
As a second example radio realized embodiment illustrated in FIG. 2B, the LTE transmissions could be multiplexed with the GSM transmissions on GSM burst level. GSM transmits speech using bursts, each with a duration of 0.577 ms. In speech operation, after having sent one burst, the Rx/Tx part sleeps for 7*0.577 ms until it wakes up again and do a new Rx/Tx process. In this second example this time gap could be used for LTE transmissions.
As a third example radio realized embodiment illustrated in FIG. 2C, any of above can be used for transmission but by using dual receiver for simultaneous reception of GSM and LTE in the downlink for simplified operation.
The architecture and principles of the circuit switched fallback (CSFB) are defined in, e.g., 3GPP TS 23.272, Circuit Switched Fallback in Evolved Packet System, Stage 2 (Release 8), which is abbreviated herein as “23.272” and which is incorporated herein by reference in its entirety. FIG. 3 shows example architecture for implementing an example embodiment of circuit switched fallback (CSFB). In the circuit switched fallback (CSFB) architecture and mode of FIG. 3 the following principles apply:                Reuse the combined mobility management (MM) and serving gateways (SGs) interface in SAE/LTE as defined for CS fallback [23.272].        The UE camps on LTE in CS Idle mode, CS paging and short message service (SMS) is sent via SGs and a mobility management entity (MME) as defined for CS fallback [23.272].        Circuit switched (CS) non-access stratum (NAS) signaling over Global System for Mobile communication (GSM) in active Mode as defined for CS fallback [23.272].        In active mode UE or network coordinate gaps in LTE GSM speech is allowed to be interleaved with LTE (as described, e.g., in PCT/SE2007/000358).        Use the CS Fallback architecture and principles but do not release the LTE access when tuning to GSM to make or receive calls.        
With architecture of the example type shown in FIG. 3 and assumptions the call setup performance will be similar to normal CS fallback (typically 0.5-2 s additional call setup time).
The call flows in the CS fallback specification [23.272] can be used to illustrate how the call establishment is impacted by measurements, tuning and reading of system information impacts the call setup delay. Similar call flows would be needed for the CSFB based architecture for “interleaving GSM voice in LTE” and the same issues with delays would appear.
FIG. 4 shows Mobile Originating call in Active Mode—PS HO supported (see, e.g., 3GPP TS 23.272: FIG. 6.2-1: CS Call Request in E-UTRAN, Call in GERAN/UTRAN). In other words, FIG. 4 represents a case for mobile-originated (MO) calls in active mode in which packet switched (PS) handover is supported. The call flows for mobile-terminated (MT) calls and UE's in idle mode is similar. The extra call setup delay compared to a normal call in 2G/3G are the steps 1, 2, and 3 of FIG. 4, where step 2 (measurements) is estimated to take roughly 1 s. As described in the figures, step 2 is optional (configurable in the eNodeB) but many operators indicate that they need to perform measurements in order to select the target cell.
FIG. 5 shows Mobile Originating call in Active Mode—No PS HO support (see 3GPP TS 23.272: FIG. 6.3-1: CS Call Request in E-UTRAN, Call in GERAN/UTRAN without PS HO). In other words, FIG. 5 represents a case for mobile-originated (MO) calls in active mode in which packet switched (PS) handover is not supported. The call flows for mobile-terminated (MT) calls and UE's in idle mode is similar. The extra call setup delay compared to a normal call in 2G/3G are the steps 1-8 of FIG. 5, where step 2 (measurements) is estimated to take roughly 1 s. As described in the figures, step 2 of FIG. 5 is optional (configurable in eNodeB) but many operators indicate that they need to perform measurements in order to select the target cell. In this case there is also is extra delays in between step 3 and 6 where the UE tunes to the target cell. When packet switched handover (HO) is not used this takes longer since less information is provided to the UE and the resources are not prepared in the target system. For example when RRC connection release with redirect is used, the UE needs to reselect a new cell according to the redirect information, tune to the cell and read system information (which may take a second or more).